Module mod_negotiation

Summary

Content negotiation, or more accurately content selection, is
the selection of the document that best matches the clients
capabilities, from one of several available documents. There
are two implementations of this.

A type map (a file with the handler
type-map) which explicitly lists the files
containing the variants.

A MultiViews search (enabled by the MultiViews Option, where the server does an
implicit filename pattern match, and choose from amongst the
results.

Directives

Type maps

A type map has the same format as RFC822 mail headers. It
contains document descriptions separated by blank lines, with
lines beginning with a hash character ('#') treated as
comments. A document description consists of several header
records; records may be continued on multiple lines if the
continuation lines start with spaces. The leading space will be
deleted and the lines concatenated. A header record consists of
a keyword name, which always ends in a colon, followed by a
value. Whitespace is allowed between the header name and value,
and between the tokens of value. The headers allowed are:

Content-Encoding:

The encoding of the file. Apache only recognizes
encodings that are defined by an AddEncoding directive.
This normally includes the encodings x-compress
for compress'd files, and x-gzip for gzip'd
files. The x- prefix is ignored for encoding
comparisons.

Content-Language:

The language of the variant, as an Internet standard
language tag (RFC 1766). An example is en,
meaning English.

Content-Length:

The length of the file, in bytes. If this header is not
present, then the actual length of the file is used.

Content-Type:

The MIME media type of the document, with optional
parameters. Parameters are separated from the media type
and from one another by a semi-colon, with a syntax of
name=value. Common parameters include:

level

an integer specifying the version of the media type.
For text/html this defaults to 2, otherwise
0.

qs

a floating-point number with a value in the range 0.0
to 1.0, indicating the relative 'quality' of this variant
compared to the other available variants, independent of
the client's capabilities. For example, a jpeg file is
usually of higher source quality than an ascii file if it
is attempting to represent a photograph. However, if the
resource being represented is ascii art, then an ascii
file would have a higher source quality than a jpeg file.
All qs values are therefore specific to a given
resource.

Example:

Content-Type: image/jpeg; qs=0.8

URI:

The path to the file containing this variant, relative to
the map file.

MultiViews

A MultiViews search is enabled by the MultiViews Option. If the server receives a
request for /some/dir/foo and
/some/dir/foo does not exist, then the
server reads the directory looking for all files named
foo.*, and effectively fakes up a type map which
names all those files, assigning them the same media types and
content-encodings it would have if the client had asked for one
of them by name. It then chooses the best match to the client's
requirements, and returns that document.

If set, this directive allows content-negotiated documents
to be cached by proxy servers. This could mean that clients
behind those proxys could retrieve versions of the documents
that are not the best match for their abilities, but it will
make caching more efficient.

This directive only applies to requests which come from
HTTP/1.0 browsers. HTTP/1.1 provides much better control over
the caching of negotiated documents, and this directive has no
effect in responses to HTTP/1.1 requests.

The LanguagePriority sets the precedence of language
variants for the case where the client does not express a
preference, when handling a MultiViews request. The list of
MIME-lang are in order of decreasing preference.
Example:

LanguagePriority en fr de

For a request for foo.html, where
foo.html.fr and foo.html.de both
existed, but the browser did not express a language preference,
then foo.html.fr would be returned.

Note that this directive only has an effect if a 'best'
language cannot be determined by any other means. Correctly
implemented HTTP/1.1 requests will mean this directive has no
effect.